The invention relates to a high frequency, high pressure gas discharge lamp system including
a high pressure gas discharge lamp with a discharge vessel sealed in a gas-tight manner and enclosing a discharge space, a discharge sustaining filling, and means for maintaining a discharge within said discharge space during lamp operation; and PA1 a ballast circuit coupleable to the discharge lamp for energizing the discharge lamp to maintain a gas discharge within the discharge space during lamp operation. PA1 the discharge lamp during normal lamp operation is free of acoustic resonances at alternating lamp currents below a lowest lamp resonant frequency, PA1 the ballast circuit energizes the discharge lamp so as to have an alternating lamp current having a fundamental frequency and harmonics which are integral multiples of the fundamental frequency, PA1 the fundamental frequency and the lowest lamp resonant frequency are greater than about 19 kHz, and PA1 the harmonics above the lowest lamp resonant frequency have amplitudes which are insufficient to induce acoustic resonance.
Such a system is known from the article "An Autotracking System For Stable Hf Operation of HID Lamps", F. Bernitz, Symp. Light Sources, Karlsruhe 1986.
Gas discharge lamps have traditionally been operated with low frequency magnetic ballasts. High frequency ballasts are becoming increasingly popular for low pressure mercury vapor fluorescent lamps. For fluorescent lamps, high frequency operation significantly improves lamp efficacy and permits the magnetic elements of the ballast to be greatly reduced in size and weight compared to conventional magnetic ballasts. Similar reduction in size and weight would be desirable for HID lamps, especially for lower wattage metal halide lamps used for shop and track lighting, because it would provide greater flexibility in designing aesthetically pleasing fixtures for such uses. System efficiency would also increase a few percent, though not nearly as much as for fluorescent lamps, due to lower ballast losses.
A major obstacle to the use of high frequency electronic ballasts for HID lamps, however, is the acoustic resonances/arc instabilities which can occur at high frequency operation. Acoustic resonances, at the minimum, can cause flicker of the arc which is very annoying to humans. In the worst case, acoustic resonance can cause the discharge arc to extinguish, or even stay permanently deflected against and damage the wall of the discharge vessel, which may cause the discharge vessel to rupture.
The above-cited article discloses a ballast which continuously varies the lamp operating frequency about a center frequency over a sweep range. The sweep frequency is the frequency at which the operating frequency is repeated through the sweep range. The ballast senses lamp voltage to evaluate arc instabilities. A control signal is derived from the sensed lamp voltage to vary the sweep frequency between 100 Hz and some kHz to achieve stable operation. However, this system has never been commercialized.
A disadvantage of such a system is that changing the operating frequency and/or sweeping the operating frequency over a range of operating frequencies requires additional control components for these functions which increases the size and cost of the ballast. Additionally, these changes would change the lamp power, requiring still further control mechanisms, for example adjusting the DC-supply voltage or the current limiting inductance, to maintain the desired power.